1. Field of the Invention
The present invention relates to a control system for a commutatorless motor, in which a synchronous motor is driven by a frequency converter comprising thyristors, and more particularly to a control system for a commutatorless motor, which is free from a change in response in the speed control loop caused by a change in a load and therefore capable of control with highly reliable response.
2. Description of the Prior Art
A commutatorless motor, as well known, is a variable speed motor which is a combination of a synchronous motor and a frequency converter comprising thyristors for controlling the motor and which has no commutator and brush. The commutatorless motor is advantageous in that it has a very wide range of controllable speed and that its lack of commutator and bursh facilitates maintenance and check.
In case where a load with large fluctuation such as a steel rolling mill is driven by such a commutatorless motor, the load-dependent current intermits when the load shifts to a light level. As well known, the gain of the current control circuit provided as a minor loop in the speed control loop, largely varies in the continuous range (where the load-dependent current is continuous) and in the discontinuous range (where the load-dependent current intermits). Namely, the gain of the current control circuit falls to a great extent in the discontinuous range. However, since the response characteristic of the speed control loop must be adjusted in accordance with that in the discontinuous range, the response characteristic in the continuous range is therefore liable to be degraded and unstable. Consequently, it is difficult to obtain a satisfactory characteristic of control.